1. Field of the Invention
This invention relates to a display apparatus of the active matrix type wherein a light emitting element is used in a pixel and a driving method for a display apparatus of the type described. The present invention relates also to an electronic apparatus which includes a display apparatus of the type described.
2. Description of the Related Art
In recent years, development of a display apparatus of the planar self-luminous type which uses an organic EL (electroluminescence) device as a light emitting element is proceeding energetically. The organic EL device utilizes a phenomenon that, if an electric field is applied to an organic thin film, then the organic thin film emits light. Since the organic EL device is driven by an application voltage lower than 10 V, the power consumption of the same is low. Further, since the organic EL device is a self-luminous device which itself emits light, it requires no illuminating member and can be formed as a device of a reduced weight and a reduced thickness. Further, since the response speed of the organic EL device is approximately several μs and very high, an after-image upon display of a dynamic picture does not appear.
Among display apparatus of the flat self-luminous type wherein an organic EL device is used in a pixel, a display apparatus of the active matrix type wherein thin film transistors as active elements are formed in an integrated relationship in pixels is being developed energetically. A flat self-luminous display apparatus of the active matrix type is disclosed, for example, in Japanese Patent Laid-open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791 and 2004-093682.
FIG. 16 schematically shows an example of an existing active matrix display apparatus. Referring to FIG. 16, the display apparatus shown includes a pixel array section 1 and a peripheral driving section. The driving section includes a horizontal selector 3 and a write scanner 4. The pixel array section 1 includes a plurality of signal lines SL extending along the direction of a column and a plurality of scanning lines WS extending along the direction of a row. A pixel 2 is disposed at a place at which each of the signal lines SL and each of the scanning lines WS intersect with each other. In order to facilitate understandings, only one pixel 2 is shown in FIG. 16. The write scanner 4 includes a shift register which operates in response to a clock signal ck supplied thereto from the outside to successively transfer a start pulse sp supplied thereto similarly from the outside to output a sequential control signal to the scanning line WS. The horizontal selector 3 supplies an image signal to the signal line SL in synchronism with the line sequential scanning of the write scanner 4 side.
The pixel 2 includes a sampling transistor T1, a driving transistor T2, a storage capacitor C1 and a light emitting element EL. The driving transistor T2 is of the P-channel type, and is connected at a source thereof, which is one of current terminals, to a power supply line and at the drain thereof, which is the other current terminal, to the light emitting element EL. The driving transistor T2 is connected at the gate thereof, which is a control terminal thereof, to the signal line SL through the sampling transistor T1. The sampling transistor T1 is rendered conducting in response to a control signal supplied thereto from the write scanner 4 and samples and writes an image signal supplied from the signal line SL into the storage capacitor C1. The driving transistor T2 receives, at the gate thereof, the image signal written in the storage capacitor C1 as a gate voltage Vgs and supplies drain current Ids to the light emitting element EL. Consequently, the light emitting element EL emits light with luminance corresponding to the image signal. The gate voltage Vgs represents a potential at the gate with reference to the source.
The driving transistor T2 operates in a saturation region, and the relationship between the gate voltage Vgs and the drain current Ids is represented by the following characteristic expression:Ids=(½)μ(W/L)Cox(Vgs−Vth)2 where μ is the mobility of the driving transistor, W the channel width of the driving transistor, L the channel length of the driving transistor, Cox the gate insulating layer capacitance per unit area of the driving transistor, and Vth is the threshold voltage of the driving transistor. As can be apparently seen from the characteristic expression, when the driving transistor T2 operates in a saturation region, it functions as a constant current source which supplies the drain current Ids in response to the gate voltage Vgs.
FIG. 17 illustrates a voltage/current characteristic of the light emitting element EL. In FIG. 17, the axis of abscissa indicates the anode voltage V and the axis of ordinate indicates the drain current Ids. It is to be noted that the anode voltage of the light emitting element EL is the drain voltage of the driving transistor T2. The current/voltage characteristic of the light emitting element EL varies with time such that the characteristic curve thereof tends to become less steep as time passes. Therefore, even if the drain current Ids is fixed, the anode voltage or drain voltage V varies. In this regard, since the driving transistor T2 in the pixel circuit 2 shown in FIG. 16 operates in a saturation region and can supply drain current Ids corresponding to the gate voltage Vgs irrespective of the variation of the drain voltage, the emission light luminance can be kept fixed irrespective of the time-dependent variation of the characteristic of the light emitting element EL.
FIG. 18 shows another example of an existing pixel circuit. Referring to FIG. 18, the pixel circuit shown is different from that described hereinabove with reference to FIG. 16 in that the driving transistor T2 is not of the P-channel type but of the N-channel type. From a fabrication process of a circuit, it is frequently advantageous to form all transistors which compose a pixel from N-channel transistors.